Visited Santorini earlier this year – beautiful place. Temperatures on average during the day were about 25C in direct sunlight. I noticed a lot of the buildings/railings are coated in a white paint and surprisingly were cool to the touch. Like, not just comfortable by straight up cold even in the beating sun. This can’t just be part of the natural reflective qualities of using white, can it?
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It can be, with the right paint.
Things can heat up either with contact with something hot (and conversely cool down by touching something cold) or by absorbing radiation.
So if a white paint reflects enough sunlight (visible and infrared) that it sheds more heat than it absorbs, it can even be cooler than surrounding air. Put it on something that can absorb lots of heat before noticeably raising in temperature (say, over the course of the day), like stone or metal, and the surface will feel cool even with a middling white paint. Because it’s cooled from one side but not heated fast enough from the other.
Stone has a high thermal mass (which means it can store a lot of heat energy) and a high thermal conductivity (which means if you touch it then it transfers heat very quickly).
In this situation there are two ways in which the stone can heat up. Radiation heat from the sun & from the ambient temperature of the surrounding air.
White paint means that most of the UV radiation is reflected away, and air itself is a very poor thermal conductor.
So even though it gets very hot in the day, it would take a long time for the air temperature alone to heat the stone up, & since it gets cool or cold at night, the stone then loses some of that heat energy it gains during the day time.
And finally since stone is a good conductor of heat, when you touch it then it feels cool to the touch as heat is being conducted from your hand into the stone.
You can see a similar effect on the beach on a very hot day, the ocean has a very high thermal mass so it takes a long time to heat up, the land heats much more quickly. Because the land is hot, the air over it rises up and is replaced with cold air from the sea. Which is why on a very hot day you often get quite a cold breeze blowing onto land from the sea even though a mile inland its extremely hot.
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